Polyethylene composition for use in insulations and joints of extra-high voltage power cables, and an extra-high voltage power cable and joint therefor employing this polyethylene composition

Abstract

A polyethylene composition which is useful in insulations and joints of extra-high voltage power cables, and in which a cross-linking agent, comprising an organic peroxide subjected to purification processing, and an antioxidant, are blended, wherein the cross-linked composition has a dielectric loss tangent (tan delta ) of less than or equal to 0.07% at a temperature of 90 DEG C. and under an electric field stress of 20 kV/mm. This polyethylene composition exhibits little dielectric loss, even at high temperatures or high electric field stresses, and is optimally employed in insulations and joints for extra-high voltage power cables of 275 kV or more.

What is claimed is: 
     
       1. A polyethylene composition for use in insulations and joints for extra-high voltage cables, comprising an organic peroxide cross-linked polyethylene and an antioxidant, having a dielectric loss tangent (tanδ) of 0.07% or less at a temperature of 90° C. and at an electric field stress of 20 kV/mm. 
     
     
       2. A polyethylene composition for use in insulations and joints for extra-high voltage cables in accordance with claim 1, wherein said polyethylene composition is such that a relationship between a dielectric loss tangent (tanδ 2 ) and volume resistivity (ρ 2 ) of the cross-linked composition, wherein a cross-linking agent and an antioxidant are blended into a low density polyethylene, after being subjected to extraction processing with an organic solvent, and a dielectric loss tangent (tanδ 1 ) and volume resistivity (ρ 1 ) of said composition prior to extraction, is such that, under conditions of a temperature of 90° C. and a stress of 20 kV/mm,   tanδ.sub.1 /tanδ.sup.2 ≲5     and/or     ρ.sub.2 /ρ.sub.1 ≲5.     
     
     
       3. A polyethylene composition for use in insulations and joints of extra-high voltage cables in accordance with claim 1, wherein said polyethylene composition is such that a value obtained when a volume resistivity of an extract obtained when 10 g of a composition, in which 1-5 parts by weight of a cross-linking agent comprising an organic peroxide subjected to purification processing and 0.05-0.5 parts by weight of an antioxidant are blended into 100 parts by weight of low density polyethylene and this is cross-linked, is treated with 10 ml of nonpolar solvent, its value multiplied by a viscosity of said extract, is greater than or equal to 5×10 10  ΩPa/m. 
     
     
       4. A polyethylene composition for use in insulations and joints of extra-high voltage cables in accordance with claim 1, wherein said polyethylene composition is such that a value obtained when an amount of increase in conductivity of an extract obtained when 10 g of a composition, in which 1-5 parts by weight of a cross-linking agent comprising an organic peroxide subjected to purification processing and 0.05-0.5 parts by weight of an antioxidant are blended in 100 parts per weight of low density polyethylene and this is subjected to cross-linking, is treated with 10 ml of a polar solvent, its value divided by the viscosity of the extract, is less than or equal to 5×10 -5  S/Pams. 
     
     
       5. A polyethylene composition for use in insulations and joints of extra-high voltage cables in accordance with claim 1, wherein said cross-linking agent comprises dicumyl peroxide, and an oil obtained by dissolving 2 parts by weight of said dicumyl peroxide in 100 parts by weight of a Type 1-Number 1-alkyl benzene oil in accordance with JIS-C-2320 having a volume resistivity of 1×10 15  Ω·cm or more, which volume resistivity is calculated from an insulating resistance after the passage of 1 minute obtained in accordance with JIS-C-2101 (electrically insulating oil test method) under conditions of 80° C. and DC 500 V, has a volume resistivity of 1×10 14  Ω·cm or more as calculated from an insulating resistance after the passage of 1 minute obtained in accordance with JIS-C-2101 under conditions of 80° C. and DC 500 V. 
     
     
       6. A polyethylene composition for use in insulations and joints of extra-high voltage cables in accordance with one of claim 1 wherein said antioxidant is such that an insulating resistance of an electrically insulating oil which is obtained after dissolving 0.1 g of said antioxidant in 100 ml of electrically insulating oil having an insulating resistance of 1×10 16  Ω·cm or more, is 1×10 14  Ω·cm or more. 
     
     
       7. An extra-high voltage cable, having an insulating layer comprising one of said polyethylene compositions in accordance with one of claims 1 through 6. 
     
     
       8. A joint for extra-high voltage cables, comprising one of said polyethylene compositions in accordance with one of claims 1 through 6.